press your thumb and index finger together on your right hand. With the left, touch the fleshy "ball" of your thumb, that is what a rare steak feels like.
For medium rare, it's index and second finger. Medium is index, second and third. Anything after that is well done (inedible as far as I'm concerned!) Above is based on thick cuts (1 inch or >) of rib-eye, striploin or fillet that have been hammered a little bit with your fist, not a hammer.

For tougher stuff like plain rib, round or sirloin, I would use the same rule of "thumb" but unless you've been able to age the meat for a few days, I would use a meat hammer it until it's twice its original size and only half as thick. Some purists might want to string me up for that but it works really well and gets you a fantastically soft steak once its been stood for 5 minutes after cooking. It will shrink during cooking anyway.

Agree with Bock that the steak should come off a grade before what you want. And always make sure it the pan is smoking hot before you put it on.

You must go by the firmness of the steak.

You should always oil the steak, not the pan, and you should make sure to sear it on both faces and along the edge. Make sure the steak is at room temperature before searing it. Take it out of the fridge an hour before cooking.

Rare steak has the consistency of flesh and droplets of reddish juice at the surface.

When it's medium-rare, the meat springs back when you touch it with your finger.

Medium steak is qute firm to the touch with pinkish juice.

Well-done meat feels quite stiff with brown juices.

Steak continues to cook after you remove it from the pan, so I'd cook it to one level down from the degree of cooking you want. Let it rest for five minutes or so to let the juices redistribute and return it to the pan for a few seconds each side before serving.

Now, any tips on how to judge when the steak is cooked to your liking, I usually err on the side of caution and lob it back in the pan if underdone, which is fine when cooking for myself and the missus but the knife cuts used to inpect it don't look great when I'm entertaining (not that that happens too often)